Metal casing is commonly used in oil well boreholes, and it is desirable to periodically determine the physical condition and integrity of the casing, which is subject to deterioration, such as from corrosion. Ultrasonic inspection of casing and other piping is known in the art. One type of such equipment is utilized by the assignee of the present application, Schlumberger Technology Corporation, and is called the Ultrasonic Imager ("USI"--trademark of Schlumberger Technology Corporation). In an example of a borehole ultrasonic inspection equipment, a tool is lowered in a cased borehole and has a rotating acoustic transducer that emits a pulse of ultrasonic energy toward the casing. As shown, for example, in U.S. Pat. No. 5,274,604, which relates to characterizing interfaces formed between various materials in a cased borehole, the transducer can be focused. The echoes from the casing are received by the same transducer, and converted to electrical signals by the transducer. The signals can be processed to obtain characteristics of the casing, including its inner radius, reflectivity, and thickness.
An accurate determination of the casing inner radius can be obtained by processing the received echoes using a "center of energy" ("COE") technique, as described, for example, in Stanke and Liang, "Profiling High-Angle Surfaces With Focused Transducers And Time-of-Flight Measurements", IEEE 1990 Ultrasonics Symposium, 1990. However, existing techniques of casing thickness determination could stand improvement. The reflected echoes from the casing outer surface tend to be small compared to those from the inner surface. Also, the consistent detection of the arrival of echoes from the casing outer surface can be difficult. When the ultrasound energy first impinges on the casing inner surface, both compressional and shear ultrasonic components propagate toward the casing outer surface, and when some of the energy from these components reflect off the casing outer surface, both compressional and shear components are again generated and propagate back toward the casing inner surface, with energy therefrom being ultimately received by the transducer. Compressional (p) components have a substantially higher velocity than shear (s) components. In general, the casing thickness would be ideally determined from the initial p-p echo [the forward and reflected compressional components, which arrive first]. The p-s and s-p components arrive at the transducer at about the same time and can have a cumulatively greater amplitude than the somewhat earlier p-p arrival. Although the p-p arrival can usually be distinguished from the later arriving p-S/s-p arrivals, the p-p can also be confused with the ringing tail end of the main (first) reflection from casing inner surface. This is particularly true for thin casings and for reflections from casing outer surface pits and other deformities.
Further limitations of existing ultrasonic casing inspection systems relate to their ability to obtain relatively high resolution measurements of casing characteristics at a relatively high rate, and to communicate sufficient information to the earth's surface on a limited bandwidth communications channel.
It is among the objects of the present invention to provide solutions to the above-indicated problems and limitations of the prior art, and to generally improve ultrasonic inspection of casing and other piping.